Method of electroplating zinc and electroplating compositions



United States Patent METHoD or ELEQTROP ATIN ZI C AND ELECTROPLATINGCOMPOSITIONS Allan E. Chester, Highland Park, and Ray F. Main, Chicago,11].; said Chester, assignor to Poor & Company, Chicago, 111., acorporation of Delaware, and said Main, assignor to Acme Steel Company,Chicago, 111., a corpoi'ation of Illinois No Drawing. Application April15, 1954,

Serial No. 423,538

7 Claims. (Cl. 204-55 This invention relates to a new 'and improvedmethod of electroplating zinc from an acid "zinc bath and to a new andimproved electrolyte therefor.

One of the objects of the invention is "to provide a new and improvedmethod for electroplating zinc from acid zinc plating baths.

Another object of the invention is to provide a new and improved methodfor electrodepositing zinc in finely crystalline (fine grain) form froman electroplating bath which operates over a relatively wide range ofpH, zinc content, current density and temperature.

Still another object of the invention is to provide a new and improvedmethod for imparting luster or brightness to zinc coatings withouttreeing and pitting by a process in which the bright coating is formeddirectly in an acid zinc sulfate plating bath.

Another object of the invention is to provide a new and improved methodfor the continuous plating of zinc from an acid zinc sulfate bath.

An additional object of the invention is to provide a new and improvedmethod for the continuous plating of zinc from an acid sulfate bath inwhich zinc is electrodeposited on the sheet or other object to be platedusing a high current density on one side of the sheet or other objectand a low current density on the other side and then reversingthe sides,if desired.

Another objectof the invention is to provide a method of electroplatingwherein bright, lustrous, fine grained, ductilezinc deposits having apleasing silvery color are deposited over a wide rangeof currentdensity.

Still a further object of the invention is to provide a method ofproducing bright zinc deposits directly in an acid zinc sulfate platingbath, simply and economically.

Another object of the invention is to provide new and usefulelectroplating compositions. Other objects will appear hereinafter. Inaccomplishing these objects in accordance with the invention it has beenfound that new and improved results -in the production of bright zincdeposits from an acid z'inc sulfate electroplating bath are obtained bycarrying out the electrodeposition of zinc from said bath in thepresence of a peptone dissolved in said bath. The bath also preferablycontains a' substance referred to herein as a carrier which serves toproduce a fine grained zinc "deposit. The carrierrpreferably employedfor the purpose of the invention is thiourea with -or without zincgluco- 'nate, sodium fluoride and/ or aluminum'sulfate.

The peptones preferably employed for the purpose of the invention aremixtures ofproteoses and amino acids derive d bythe enzymatic digestionof materialsof animal origin, e. g., glatimanimal stomach linings, andthe like. Peptones can be made, for example, by starting with gelatinmade from animal hoofs and preparing a nutrient broth to which is addeda peptone-forming enzyme which acts on the proteins to form peptones. Aspecific peptone especially suitable for the practice of the presentinven- Patented July 16, 1957 tion is Peptone No. 421 which is a viscousbrown liquid made by Wilson & Company.

The invention will be further illustrated but is not limited by thefollowing example in which the quantities are stated in parts by weightunless otherwise indicated.

Example An acid zinc plating bath is prepared by dissolving 1 pound to 3pounds of ZnSO4.7H2O in water and adding suflicient sulfuric acid togive a pH within the range of -l to 5.5, preferably 3 to -4. The bathpreferably has a zinc content of .75 pound zinc per gallon of solution.A carrier is prepared by dry mixing the following ingredients:

Ingredients: Percent by weight Zinc gluconate 4.71 Thi'o'urea 23.53Sodium fluoride (commercial) l8.82 Aluminum sulfate (hydrated) 52.94

The carrier is added to the above mentioned acid zinc sulfate solutionin proportions preferably within the range of A; to 2 ounces of carrierper gallon of solution. This corresponds to about 0.28 to 4.48 grams ofthiourea per liter of electrolyte.

Peptone No. 421 (Wilson & Company) is added to the previously describedsolution containing the carrier, preferably in proportions within therange of 0.075 to 0.5 cc. per liter of solution.

Steel strips or sheets are plated in this bath by passing themcontinuously through the bath. The current density may vary from about 3amperes per square foot to several thousand amperes per square foot butthe bath is usually run at an average current density of 300 amperes persquare foot. It will be recognized, of course, that the current densityon the edges of steel sheets or strips may be higher, say around 1000amperes per square foot, "-When'the average. current density is 300amperes per square foot.

The carrier 'is not, or is only partially, destroyed by the passage ofthe current but in continuous plating operations it is necessary to addadditional quantities of carrier in orderto replace the quantities whichare removed by dragout. Thus, if the bath contains 16 pounds of thecarrier per 1000 gallons of solution, then '16pounds of the carriershould be added for each 1000 gallons of solution lost by dragout. Sincethe quantity of the carrier is not extremely critical, it is usuallypossible to keep a satisfactory balance in the solution by making adaily addition of carrier equivalent to the loss by dragout. The bathcancontain at leasttwice the normal concentration ofcarrierwithout-sufiering a bad plate.

The amount peptone added to the solution is controlled by the use of aHull cell in which the temperature of the bath is maintained at F. andthe solution is kept moving on the surface of the plate during theplating operation 'bymeans of compressed air introduced below "thebottom edge of the plate. The Hull cell when operatedat 1 ampere for 10minutes gives great contrast between the bright and dark zone becausethe bright zone in :a properly balanced bath'will be almost mirrorbright. The quantity of brightener added to the bath is prefer- "ablysuch as'to maintain a bright zone on the Hull cell that extends below 3amperes per square foot. The presence of m'inuteamounts of lead in thebath resist the extension 'ofthe bright zone downwardly and make itdifficult toreach below 3 amperes per square foot. The bath isconsidered to be best when the lead content is -below 1 part permillion.

The Hull cell test also gives an indication of carrier requirements. Ifthe quantity of carrier in the bath is low, it will shorten the brightrange on the Hull cell plate and if the quantity of carrier is in excessof the optimum amount, the bright zone on the Hull cell plate isdarkened and divided into several zones and shows a graining at the highcurrent density end. For the control of the operations in continuousplating, it-ispreferable to .use at least one Hull cell test per dayiandmake additions of the peptone suificient to extend the bright rangeinthe Hull cell below 3 amperes per square foot. Occasional checks aremade by adding some of the carrier to the Hull cell and if an extensionof the bright range is encountered, the indicated amount of carrier isadded to the main bath. Ingeneral, when larger amounts of'the peptoneare employed, it is desirable to use a greater amount. of carrier inorder to extend the bright range to the lower current densities andsecure the desired result in the plating operation. For example, whenusing 0.075 cc. per liter of the peptone material, it is desirable toemploy about-0.7 gram per liter of thiourea or a sufiicient amount ofthe carrier composition previously described to furnish thisquantity ofthiourea. On the other handfwhen employing 0.5 cc. of the peptonematerial per liter of plating solution, it is desirable to use about 1to 1.25 gram of thiourea per liter of plating solution or a quantity ofthe previously described carrier composition suflicient to furnish thisamount of thiourea.

The thiourea employed in the above described plating solution assists inproducing a fine grained zinc plate. Without the thiourea, the desiredbrightening effect is not obtained with the peptone material.Apparently, the peptone material acts as an interference brightener.

The zinc gluconate has pronounced anti-burn efiiects and the combinationof zinc gluconate and thiourea in itself has some brightening action butnot to the extent of that obtained when the peptone material is alsoadded to the bath.

The aluminum sulfate is preferably added to the bath in quantities fromA to 2 ounces per gallon and apparently serves to form a film ofcolloidal alumina in the vicinity of the anode and prevent excessiveacid attack upon the zinc anode.

Sodium fluoride acts as a whitening and grainrefining agent at thecathode and apparently causes more uniform solution of zinc at the anodewhile tending to prevent sludge formation. 1

Other optional ingredients which may be added to the bath are sugars,such'as dextrose, molasses and the like, which apparently produce arefinement in the crystalline structure of the zinc and have somewhitening effect. Other ingredients, such as sodium acetate, may beincluded in the bath but all of these auxiliary ingredients areoptional. I

The compositions employed in the practice of the invention areespecially effective in the electrodeposition of zinc from acidic bathsin which the zinc is principally present as zinc sulfate; The zincsulfate baths can also contain additions of zinc chloride, preferablynot more than 20 to 100 grams per liter.

The invention provides a new and improved method for electrodepositingzinc in finely crystalline form from an electroplating bath whichoperates over a relatively wide range of pH, zinc content, currentdensity and temperature. The invention also provides a new and improvedmethod for imparting luster or brightness to zinc coatings withouttreeing and pitting and by a process in which the bright coating isformed directly in an acid zinc plating bath without secondarytreatments. The invention is especially suitable for continuous platingof steel strips or sheets from an acid zinc sulfate bath in which zincis electro-deposited on the sheet or other object to be plated using ahigh current density on one side of the sheet or other object and a lowcurrent density on the other side and then reversing the sides. Thequantities of the additives required in order, to produce bright,lustrous, fine grained, ductile zinc deposits having a pleasing silverycolor over a relatively wide range of current densities are extremelysmall.

The term acid zinc sulfate plating bat is used herein to describe acidicplating baths in which the zinc is present principally as zinc sulfatebut can also be present in minor amounts in one or more other forms.

The invention is hereby claimed as follows:

1. An aqueous acid zinc sulfate plating bath comprising an acid zincsulfate eectrolyte, a quantity of thiourea sufficient to produce a finegrained ductile zinc deposit when zinc is electrodeposited from saidelectrolyte and a quantity of a peptone sufficient to enhance thebrightness of said deposit.

2. An aqueous acid zinc sulfate plating bath comprising an acid zincsulfate electrolyte, about 0.28 gram to about 4.48 grams of thiourea perliter of said electrolyte and about .075 to about 0.5 cc. of a peptoneper liter of said electrolyte.

3. In a process for the electrodeposiiton of zinc, the step whichcomprises electrodepositing zinc from an acid zinc sulfate plating bothcontaining a suflicient quantity of thiourea to refine the grain size ofthe electrodeposited zinc and a suflicient quantity of a peptone toenhance the brightness of the electrodeposited zinc.

4. In a process for the electrodeposition of zinc, the step whichcomprises electrodepositing zinc from an acid zinc sulfate plating bathcontaining thiourea, a peptone and zinc gluconate, the quantity ofthiourea being sufiicient to refine the grain size of theelectrodeposited zinc and the quantity of the peptone being suflicientto enhance the brightness of the electrodeposited zinc.

5. In a process for the electrodeposition of zinc, the step whichcomprises electrodepositing zinc from an acid zinc sulfate plating bathcontaining thiourea, a peptone, zinc gluconate and aluminum sulfate, thequantity of thiourea being suflicient to refine the grain size of theelectrodeposited zinc and the quantity of the peptone being suflicientto enhance the brightness'of the electrodeposited zinc.

6. In a process for the electrodeposition of zinc, the

step which comprises electrodepositing zinc from an acid which compriseselectrodepositing zincfrom an acidzinc sulfate plating bath containingabout .075 to 0.5 cc. of a peptone per liter of said bath and about 0.28gram to 4.48 grams ofthiourea per liter of said bath, the quantity ofthiourea being low when the quantity of peptone is low, and the quantityof thiourea. being high when the quantity of peptone is high, andreplenishing at intervals the quantities of said materials which areremoved from said bath by dragout;

References Cited in the file, of this patent UNITED STATES PATENTSChester et al. Oct. 25, 1949 Chester et al Oct.' 3, 1950 Chester .Oct.21, 1952 OTHER REFERENCES Sato; Chemical Abstracts, vol. 24 (1930), pp.3445. Diggin: Metal Industry, June 25, 1943, p. 385. Dobbs: MetalIndustry, Oct. 22,1926, p. 385

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,2,799,635 Jul; 16, 1957 Allan E, Chester et al.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 1, line 37, for acid sulfate" read acid zinc sulfate column 4,line 21, for "electrodeposiiton" read electrodeposition line 23, for"both read bath line 45, after "aluminum" insert sulfate same column 4,line 69, list of references cited, under "OTHER REFERENCES" for "Diggin:Metal Industry, June 25, 1943, p. 385."

read

=-- Diggin: Metal Industry, June 25, 1943, p, 407.,

Signed and sealed this 21st day of January 1958a (SEAL) Attest:

KARL MINE ROBERT c. WATSON Attesting Officer Conmissioner of Patents

1. AN AQUEOUS ACID ZINC SULFATE A PLATING BATH COMPRISING AN ACID ZINC SULFATE EECTROYLTE, A QUANTITY OF THIOUREA SUFFICIENT TO PRODUCE A FINE GRAINED DUCTILE ZINC DEPOSIT WHEN ZINC IS ELECTRODEPOSITED FROM SAID ELECTROLYTE AND A QUANITY OF A PEPTONE SUFFICIENT TO ENHANCE THE BRIGHTNESS OF SAID DEPOSIT. 